Optical fibers are capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. An optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the axis of the fiber with minimum attenuation. Typical optical fibers are made of high purity silica with minor concentration of dopants to control the index of refraction.
Although optical fiber is capable of transmitting optical signals over long distances with very low loss, the size and complexity of modem telecommunication network necessitates amplification of the signal within the fiber to maintain signal accuracy and transmission efficiency. In a typical telecommunications network, light of approximately 1550 nanometers (nm) is transmitted along the guided wave portion of a waveguide, typically an optical fiber.
Due to attenuation of the light signal along the length of the optical fiber, it is necessary to reinforce or amplify this signal at given intervals along the fiber. Optical fiber amplifiers such as erbium-doped fiber amplifiers are used to intensify the optical signal that is attenuated along the fiber optic communication path. These optical fiber amplifiers have replaced cumbersome electrical repeaters in fiber optic communication links.
In a typical case, a section of an optical fiber is doped with ions of rare-earth elements such as, for example, erbium. The energy structure of the erbium ion is such that the signal light with wavelength of approximately 1530–1565 nm can be amplified in the fiber if the fiber is pumped by a suitable light source. In such a circumstance, light within the same bandwidth entering the optical fiber will experience a net gain, and will exit the fiber with greater power. One problem limiting the capacity of such systems is that the erbium-doped fiber amplifier has a characteristic spectral dependence providing different gains for different wavelengths. This spectral dependence poses a problem for multi-channel wavelength division multiplexed systems because different gains for different channels would lead to high bit error rates in some of the channels.
Typically, as illustrated by the graph depicted in FIG. 1, an erbium-doped fiber amplifier gain spectrum has a primary gain peak between 1531 and 1532 nanometers, a subsidiary maximum near 1546 and 1556 nm, and minima around 1538 and 1551 nm. To achieve the flat gain spectrum required for broadband optical fiber communications, the erbium-doped fiber amplifier can be combined with a component that has a loss spectrum shaped to compensate for the wavelength dependence of the amplifier gain. Ideally, a gain flattening component should allow matching at any time to any amplifier gain spectrum.
One approach to this problem has been to incorporate various spectral shaping devices into the optical fiber to help flatten the gain spectrum of the erbium-doped fiber amplifier. In one approach, a series of uniform long period gratings were formed in an optical fiber. Each of the gratings removed certain spectral components of the gain spectrum of the erbium-doped fiber amplifier. Unfortunately, flattening the gain spectrum of an erbium amplifier in this manner results in a relatively long optical device, on the order of several decimeters, which is difficult to manufacture and package.
In general, there are two basic ways to flatten a gain spectrum of a fiber amplifier: one way is to tailor the material properties of the erbium-doped fiber, and the other is to use filters designed to approximate the inverse characteristics of the gain spectrum of the fiber amplifier. Because a gain flattening filter requires transmission characteristics of very low back reflection and wide band width, long period grating are particularly useful.
As is well known in the art, gratings are formed using grooves or other artifices to refract light incident upon the grating. These grooves or other artifices are arranged in a repeating pattern that is characterized by a spatial period. In its broadest aspect, a fiber grating is composed of alternating regions having differing indices of refraction.
Long period gratings couple the fundamental mode of transmission of light through the fiber with cladding modes, propagating in the same direction. This is fundamentally different from Bragg fiber gratings, where the fundamental mode is coupled to modes with the opposite direction of propagation. Because the long period grating does not produce reflected light, unlike a Bragg grating, the long period grating is especially well suited as a spectrally selective attenuator.
Typically, the excited cladding mode coupled by the long period grating dissipates in the coated fiber part of the grating, which results in a resonance loss in the transmission spectrum of light passing through the grating region. Normally, over a given spectral range, the spectrum of a long period grating has only one pronounced resonance, and although changing the parameters of the grating can tailor the shape of the loss spectrum, such a grating may be used only to flatten the peak at a single wavelength of the amplifier. Thus, to flatten all the peaks in an output spectrum of a fiber amplifier, a series of long period gratings must be chained together, or the period or refractive index modulation along the grating may be varied in a single grating.
For example, in one method, a single long period grating was manufactured by introducing a phase shift by adding a length of unperturbed region in the middle of the long period grating to produce a resonance loss spectrum having two peaks. In other words, rather than having a grating where the perturbed and unperturbed regions are all the same size, at some point in the middle of the grating, the unperturbed region was longer. Alternatively, the grating may be chirped using methods well known to those skilled in the art. The disadvantage of this method, however, is that such gratings are difficult to manufacture, and efficient matching of complex spectra may not be possible.
In another approach, long period gratings may be chained together. One way to manufacture a gain flattening filter is to combine a series of long period gratings having different fundamental periods. However, the loss spectrum of each long period grating in the series may contain supplementary features, resulting in unwanted phase shift effects within the transmitted modes. These phase shifts can alter the transmission spectrum of the gain flattening filter and degrade its efficiency.
In still another approach, three short grating sections, comprising five-fifteen periods in length, were separated by long sections of fiber on the order of sixty periods. While such an arrangement can be used to flatten the gain of an optical amplifier, the resulting structure is long and requires strong modulation of the fiber's refractive index, on the order of greater than 10−3, to achieve the desired effect.
Yet another approach to flattening the gain of an optical amplifier has been to provide a coupler with a grating having a spatially varying period. However, it is difficult to control the shape of the resulting spectrum. A modification of this approach has been suggested by combining several grating sections having the same period, but each having a different effective refractive index. However, this design is impractical to manufacture because it is difficult to control the effective indices with sufficient precision. Manufacturing a filter using grating sections having the same index, but different spatial periods, is also difficult.
One problem with each of the approaches as described above is that they all use a long period grating to couple light from core of the optical fiber into a lossy mode in the cladding. This results in loss of the coupled mode into the coating of the fiber. What has been needed, and heretofore unavailable, is a mode coupling device capable of coupling between guided core mode and guided cladding modes so that the two modes co-propagate. Such a device will have a complicated spectral profile that may be employed to flatten the gain of a erbium-doped fiber amplifier. Additionally, the device should be easy to manufacture, easy to anneal, and suitable for a relatively small package compatible with commercial erbium-doped fiber amplifiers. The present invention fills these and other needs.